Some of the allegedly US euphemisms are only ever seen in the US in books written in the UK. 'Behind', of course, is used by everyone over the age of 4 for 'in back of'; young children, for some reason, are taught to use it for their backside. I have never heard 'rooster' used for the male sexual appendage, if that is what the OP is implying. 'Rooster' and 'cock' are used interchangeably for male chickens, although 12-year-old boys might giggle a little when they hear the latter.